1. Field of the Invention
The present invention relates to a method for an anti-oxidizing treatment of copper powder and, more particularly, a method for producing non-oxidizable copper powder useful as a conductive material for circuit patterns of electronic devices.
2. Description of the Prior Art
In electronic parts such as monolithic ceramic capacitors, surface acoustic wave filters and the like, it is custom to use a conductive paste to form conductive patterns of a circuit. For example, internal electrodes of monolithic ceramic capacitors are formed by applying a conductive paste on ceramic sheets by coating or printing, drying the printed sheets, and firing them in an inert atmosphere such as nitrogen atmosphere.
Such a conductive paste is generally prepared by dispersing powder of copper in an organic vehicle together with glass frit. It is very important for the conductive paste to use copper powder free from oxidation as well as to prevent it from oxidation since the oxidation of the copper powder lowers the printing property of the paste and the solderability of the produced circuit patterns and increases the sheet resistivity of produced circuit patterns. To this end, it is general practice to fire the ceramic sheets with circuit patterns of the conductive paste in an inert atmosphere to prevent copper powder from oxidation. However, the use of the inert atmosphere limits the organic vehicles that can be used for the conductive paste since it is necessary to burn out the organic vehicle during firing.
In order to burn out the vehicle, the firing atmosphere is occasionally provided with oxygen in an amount of the order of several ten to several hundreds ppm. In such a case, however, it is not possible to protect copper powder from oxidation.
In addition, since the copper powder, when being placed in air, is oxidized gradually, special care is required for storage of the copper powder. Thus, it is very troublesome to handle the copper powder.